Why do bigger discs brake better?

Home Forum Bike Forum Why do bigger discs brake better?

Viewing 40 posts - 1 through 40 (of 69 total)
  • Why do bigger discs brake better?
  • Premier Icon FOG
    Subscriber

    I was thinking about improving my front brake by putting a bigger disc on but a moments thought left me puzzled. I would be using exactly the same caliper with exactly the same pads exerting exactly the same force on exactly the same area of disc. How will this improve braking ? All I can see is that the disc should stay cooler as it would have a greater braking surface area. Can any science types shed some light on this?

    druidh
    Member

    Leverage. A bigger rotor means that the caliper and braking surface are further from the centre of rotation (axle).

    Premier Icon nedrapier
    Subscriber

    turning moments.

    try to push a door open with one finger near the hinges, then try it again with the same force, but your finger near the handle.

    Premier Icon ourmaninthenorth
    Subscriber

    Er, FOG, did you skip physics classes at school?

    Premier Icon FOG
    Subscriber

    omin, our physics teacher was nicknamed Beaky,never understood a word he said!

    julianwilson
    Member

    my (teacher) dad’s nickname at skool was also Beaky, or Captain Beaky from his more reverent pupils. He didn’t teach physic though.

    Back on topic, as above it’s about leverage. And the other thing is the larger rotor conducts more heat away from the pads and disspiates/radiates it out to the air more efficiently, meaning it is a bit further down your enormo alpine descent before your brakes overheat and stop working.

    Premier Icon Haze
    Subscriber

    Similar principal to Torque=Force*Radius

    Premier Icon GHill
    Subscriber

    Don’t forget you’ll lose some modulation too.

    Turn your bike upside down and spin the front wheel slowly.
    Stop it by poking your finger through the spoke near the rim.
    Spin it again, this time stop it by poking your finger through the spokes near the hub.

    Now do you get it ?

    robdob
    Member

    Don’t forget you’ll lose some modulation too.

    Err no you won’t.

    If anything it’ll be better as you won’t have to pull as hard to get the same braking force, making it easier to control.

    205mm discs front and back for me always. Miles better!

    cynic-al
    Member

    Haze – Member
    Similar principal to Torque=Force*Radius

    It’s exactly this principal!

    Premier Icon Haze
    Subscriber

    Thought it might be, I was relying on memory from years ago and thought it may be a little less straightforward…

    Premier Icon kiwijohn
    Subscriber

    Because they look moto

    avdave2
    Member

    So what we really need then is for someone to invent a brake that works on the rim!

    superfli
    Member

    So what we really need then is for someone to invent a brake that works on the rim!

    Buell m/c’s do or did this. Good idea, but never really that good when I tried them.

    druidh
    Member

    superfli – Member
    > So what we really need then is for someone to invent a brake that
    > works on the rim!
    Buell m/c’s do or did this. Good idea, but never really that good when I tried them.

    Whoosh!

    Even better would be one that works on the surface of the tyre.

    Trimix
    Member

    avdave2 – top idea.

    Gooner
    Member

    i am i right in thinking that with larger discs they just need less force (lever pulling) to apply the same stopping power as smaller ones?

    naedeyw
    Member

    I’m just beginning to get the whole 29er thing now…

    Premier Icon Lakes_Puma
    Subscriber

    So what we really need then is for someone to invent a brake that works on the rim!

    Buell m/c’s do or did this. Good idea, but never really that good when I tried them.

    you mean like this

    Junkyard
    Member

    I always thought it was because a larger disc had a greater circumference so therefore increased the breaking area per wheel revoloution. Does this not play arole in the process?

    Premier Icon D0NK
    Subscriber

    isn’t braking area constant ie the size of the pads? Altho I’d guess the longer circumference helps with heat dissipation

    L_P 8pot and giganto rims? that just looks silly.

    Junkyard
    Member

    yes pad area [and force applied ???] is constant. The area that the pads cover, for one turn, will be greater with a bigger rotor than a smaller one due to larger circumference. Not sure whether leverage or greater surface area is the main factor tbh

    Ax3M4n
    Member

    leverage is the main factor. Simply put – at a greater radius, less force is required to decelerate the wheel by the same amount.

    So – if it took 10 Newtons to cause a deceleration of 10 metres per second per second at a radius of 10cm, then it would only take 5 Newtons at a radius of 20cm to cause the same deceleration.

    modulation does decreace with a bigger rotor,

    Say you can apply a force of 1-10N wit your finger

    Well with a 6″ rotor you might need 10N to lock the wheel.

    With a 12″ rotor you only need 5N, so you’ve halved the ‘modulation’.

    bluebird
    Member

    I’m not disputing any of the physics, but on a bike surely isn’t it modulation and heat dissipation that are the benefits of bigger rotors.

    Premier Icon Earl_Grey
    Subscriber

    I always thought it was because a larger disc had a greater circumference so therefore increased the breaking area per wheel revoloution. Does this not play arole in the process?

    Only with respect to heat dissipation, it’s the moment of the force that has an effect (the distance from the axis of rotation).

    isn’t braking area constant ie the size of the pads?

    Doesn’t matter – it’s a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces.

    Junkyard
    Member

    cheers all

    Premier Icon D0NK
    Subscriber

    Doesn’t matter – it’s a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces.

    No physics behind it but that sounds all wrong. Bigger tyres = more grip, hold something between my hands = more grip than holding it between fingertips.

    Or am I missing something blindingly obvious?

    “it’s a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces.”

    Is that why skinny tyres are just grippy as fat tyres?

    ooOOoo
    Member

    At 20 mph, the linear velocity of the disc where it contacts the pad is greater on a 8″ disc than a 6″ disc. It travels 33% faster, and the circumference increases by 33%.

    Ignoring leverage, I can never work out what additional effect this has. Does this increased speed mean more friction?

    Perhaps it heats up quicker, but the increased surface area improves heat dissipation by a bigger factor?

    Junkyard
    Member

    now I am back to confused 😕

    cynic-al
    Member

    Bigger tyres = more grip, hold something between my hands = more grip than holding it between fingertips.

    Bigger tyres run at lower pressures give larger contact patches and more grip. Dunno re the finger one!

    Premier Icon ratherbeintobago
    Subscriber

    Ignoring leverage, I can never work out what additional effect this has. Does this increased speed mean more friction?

    Does it make any difference? The pressure (=force applied/pad area) applied will be the same (so there won’t be any additional friction assuming the same pad material); a bigger disc may have the happy side effect of dissipating heat faster due to having a bigger surface area though.

    Leverage is what’s important, I think. My mechanical physics knowledge is a bit rusty, unlike the gas laws/flow etc…

    Andy

    ac505
    Member

    To add to the confusion……if you are being pedantic then it doesn’t matter the area of the pad when thinking about contact area. A large pad with a given force will not have any more area physically in contact with the disc compared to a small pad with the same force applied. The smaller pad will have more intimate contact therefore meaning the surface area is identical.

    Premier Icon ratherbeintobago
    Subscriber

    A large pad with a given force will not have any more area physically in contact with the disc compared to a small pad with the same force applied.

    Counterintuitively, this is correct (Amonton’s 2nd Law) – in which case, why bother with 4 or 6-pot brakes? Why not use ever-bigger rotors with normal 2-pot calipers?

    Andy

    ac505
    Member

    friction has nothing to do with surface area, only coefficient of friction and normal force. However as you know heat build up/contaminants do affect the friction coefficient, so a large area pad increases the likelihood of contact and is more efficient as dissipating heat.

    Shibboleth
    Member

    *shakes head and walks away*

    Premier Icon D0NK
    Subscriber

    Oh Amonton’s 2nd Law, why didn’t you say so in the first place of course 🙄

    Bigger tyres run at lower pressures give larger contact patches and more grip

    Whilst I whole heartedly agree with you re 1.8″ vs 2.3″ on a mountain bike what about road stuff (in particular I was thinking those wide lo pro cars tyres) surely wider tyres even at the same pressure will create better grip.

Viewing 40 posts - 1 through 40 (of 69 total)

The topic ‘Why do bigger discs brake better?’ is closed to new replies.